| Autor: |
Dodd, Michael C.1, Duy Vu, Ngoc2, Ammann, Adrian, Chieu Le, Van2, Kissner, Reinhard3, Hung Viet Pham2, The Ha Cao1, Berg, Michael1, Von Gunten, Urs1 vongunten@eawag.ch |
| Předmět: |
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| Zdroj: |
Environmental Science & Technology. 5/15/2006, Vol. 40 Issue 10, p3285-3292. 8p. 3 Charts, 3 Graphs. |
| Abstrakt: |
Kinetics and mechanisms of As(III) oxidation by free available chlorine (FAC—the sum of HOCl and OCl-), ozone (O3), and monochloramine (NH2Cl) were investigated in buffered reagent solutions. Each reaction was found to be first order in oxidant and in As(III), with 1:1 stoichiometry. FAC-As(III) and O3-As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, kapp″, of 2.6 (±0.1) × 105 M-1 s-1 and 1.5 (±0.1) × 106 M-1 s-1 at pH 7, whereas the NH2Cl-As(III) reaction was relatively slow (kapp″ = 4.3 (±1.7) × 10-1 M-1 s-1 at pH 7). Experiments conducted in real water samples spiked with 50 μg/L As(III) (6.7 × 10-7 M) showed that a 0.1 mg/L Cl2 (1.4 × 10-5 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 μg/L As(III) within 10 s of oxidant addition to waters containing negligible NH3 concentrations and DOC concentrations <2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 × 10-5 M) as NH3, >75% As(III) oxidation could be achieved within 10 s of dosing 1–2 mg/L Cl2 (1.4–2.8 × 10-5 M) as FAC. As(III) residuals remaining in NH3-containing waters 10 s after dosing FAC were slowly oxidized (t1/2 ≥ 4 h) in the presence of NH2Cl formed by the FAC-NH3 reaction. Ozonation was sufficient to yield >99% depletion of 50 μg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 × 10-6 M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O3 (1.7 × 10-5 M) was sufficient for a water containing 5.4 mg-C/L of DOC. NH3 had negligible effect on the efficiency of As(III) oxidation by O3, due to the slow kinetics of the O3-NH3 reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination and ozonation of real waters were accurately modeled using the rate constants determined in this investigation. [ABSTRACT FROM AUTHOR] |
| Databáze: |
GreenFILE |
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